Anion-Exchange Membrane Water Electrolyzers

This Review provides an overview of the emerging concepts of catalysts, membranes, and membrane electrode assemblies (MEAs) for water electrolyzers with anion-exchange membranes (AEMs), also known as zero-gap alkaline water electrolyzers. Much of the recent progress is due to improvements in materials chemistry, MEA designs, and optimized operation conditions. Research on anion-exchange polymers (AEPs) has focused on the cationic head/backbone/side-chain structures and key properties such as ionic conductivity and alkaline stability. Several approaches, such as cross-linking, microphase, and organic/inorganic composites, have been proposed to improve the anion-exchange performance and the chemical and mechanical stability of AEMs. Numerous AEMs now exceed values of 0.1 S/cm (at 60–80 °C), although the stability specifically at temperatures exceeding 60 °C needs further enhancement. The oxygen evolution reaction (OER) is still a limiting factor. An analysis of thin-layer OER data suggests that NiFe-type catalysts have the highest activity. There is debate on the active-site mechanism of the NiFe catalysts, and their long-term stability needs to be understood. Addition of Co to NiFe increases the conductivity of these catalysts. The same analysis for the hydrogen evolution reaction (HER) shows carbon-supported Pt to be dominating, although PtNi alloys and clusters of Ni(OH)2 on Pt show competitive activities. Recent advances in forming and embedding well-dispersed Ru nanoparticles on functionalized high-surface-area carbon supports show promising HER activities. However, the stability of these catalysts under actual AEMWE operating conditions needs to be proven. The field is advancing rapidly but could benefit through the adaptation of new in situ techniques, standardized evaluation protocols for AEMWE conditions, and innovative catalyst-structure designs. Nevertheless, single AEM water electrolyzer cells have been operated for several thousand hours at temperatures and current densities as high as 60 °C and 1 A/cm2, respectively.


TABLES FOR HER ACTIVITY MEASUREMENTS
2 : "T.S." stands for "Tafel-slope". The values are as reported. Quotation marks are used as the majority of the reported "Tafel-slopes" were measured at a ƞ region, i.e., below ƞ < RT/F as indicated in column 7 in the Table. 3 : ƞ region used by the authors to extract the "Tafel-slope".

Table S2. HER data for Pt-Co and Pt-Ni based catalysts measured in 1 M KOH
This Table contains HER data used for Figure 12 in the paper. The majority of the data were extracted from non-steady state measurements.
Catalyst Loading Ref. 2 : j mass per total catalyst mass, i.e., A/mg cat 3 : "T.S." stands for "Tafel-slope". The values are as reported. Quotation marks are used as the majority of the reported "Tafel-slopes" were measured at a ƞ region, i.e., below ƞ < RT/F as indicated in column 7 in the Table. 4 : ƞ region used by the authors to extract the "Tafel-slope". 5 : 0.1 M instead of 1 M KOH was used.  Figure 13 in the paper. The majority of the data were extracted from non-steady state measurements.  24 1 : measured at 10 mA/cm 2 geom .
2 : "T.S." stands for "Tafel-slope". The values are as reported. Quotation marks are used as the majority of the reported "Tafel-slopes" were measured at a ƞ region, i.e., below ƞ < RT/F as indicated in column 7 in the Table. 3 : ƞ region used by the authors to extract the "Tafel-slope". 2 : "T.S." stands for "Tafel-slope". The values are as reported. Quotation marks are used as the majority of the reported "Tafel-slopes" were measured at a ƞ region, i.e., below ƞ < RT/F as indicated in column 7 in the Table. 2 TABLES FOR OER ACTIVITY MEASUREMENTS        2 : "T.S." stands for "Tafel-slope". The values are as reported. Quotation marks are used as the majority of the measurements used non-steady state methods for the evaluation.
3 : 1 M NaOH was used as electrolyte.   2 : "T.S." stands for "Tafel-slope". The values are as reported. Quotation marks are used as the majority of the measurements used non-steady state methods for the evaluation.

Electrode Preparation
In case of catalyst powders, a catalyst ink, from which an aliquot is pipetted onto a flat and inert electrode surface, is typically formed and left to dry on air and room temperature. results can be compared to results obtained from electrodes made using different ionomers, e.g., without sulfonic acid or phenolic groups or using different amounts of ionomers. It is of high importance that a thin ionomer (glue) layer is applied on top of the thin catalyst layer to ensure rapid product and reactant flow from and to the catalyst sites. This has been extensively discussed for the evaluation of the O 2 reduction reaction (ORR), which is mass S30 transport controlled. 174 Mass-transport limitations are of lower concern for the OER in thin layer electrodes, however, the guidelines established for the ORR should be followed. For accurate measurements, a real ink needs to be formed, which can be challenging.
A known amount of catalyst, which depends on the activity of the catalyst, is weight into water or a lower alcohol-based solution of known volume and sonicated for 30 to 60 min.
Catalysts can also be directly formed on the electrode. Again, in order to determine the catalyst activity, the catalyst loading and the ECSA need to be known for the determination of the mass and the intrinsic activities, respectively.

Electrolyte
Most commonly 1 M KOH is used as electrolyte for thin layer catalyst activity measurements. Some reports exist for NaOH or 0. of an AEMWE cell (e.g., using an H-cell) is glassy carbon, while gold, fluorine-doped tinoxides and sometimes also glassy carbon is used as substrate for the OER. Nickel metal could be a substrate for OER catalyst stability studies. It needs to be remembered that nickel on its own has some OER activity, which could be enhanced during the stability study by Fe incorporation and/or possible surface alterations.

Stability Measurements
CVs should be performed before and after the stability tests. OER and HER stability measurements can be performed at 1.6 V and at -0.1 V vs. RHE, respectively. However, measurements at additional potentials and the addition of cycling the potential within a narrow region also provide meaningful information (see Tables S16 and S17) and 73,180 .  (Table S18). The bare electrode substrate should also be tested using the same experimental conditions to establish the baseline. The stability measurements need to be carried out at constant temperature conditions. Higher temperatures such as 60-80 o C can be a benefit as it could reflect accelerated tests and also real operating conditions but the stability of other components such as the ionomer needs to be considered.   Table S20 provides additional experimental and operational information for the AEMWE single cell tests discussed in section 6. Table S20 complements Table S6 in the review paper. The study numbers shown in the two tables are identical.